A crop which protects the crop planted beneath it. Cereals are often used as a cover crop for newly sown grass and clover seeds. The term may also refer to crops, such as kale, which provide cover for game birds.
Broadly defined, a cover crop is any annual, biennial, or perennial plant grown as a monoculture (one crop type grown together) or polyculture (multiple crop types grown together), to improve any number of conditions associated with sustainable agriculture. Cover crops are fundamental, sustainable tools used to manage soil fertility, soil quality, water, weeds (unwanted plants that limit crop production potential), pests (unwanted animals, usually insects, that limit crop production potential), diseases, and diversity and wildlife, in agroecosystems (Lu et al.
As agroecosystems often interact with neighboring natural ecosystems in agricultural landscapes, cover crops that improve the sustainability of agroecosystem attributes may also indirectly improve qualities of neighboring natural ecosystems.
Soil Fertility Management
Cover crops are used to manage a range of soil macronutrients and micronutrients. For example in Nigeria, the cover crop Mucuna pruriens (velvet bean) has been found to increase the availability of phosphorus in soil after a farmer applies rock phosphate (Vanlauwe et al. With respect to nutrients, the impact that cover crops have on nitrogen management has received by far the most attention by researchers and farmers, because nitrogen is often the most limiting nutrient in crop production.
Cover crops known as “green manures” are grown and incorporated (by tillage) into the soil before reaching full maturity, and are intended to improve soil fertility and quality. Leguminous cover crops are typically high in nitrogen and can often, to varying degrees, provide the required quantity of nitrogen for crop production that might normally be applied in chemical fertilizer form (called fertilizer replacement value) (Thiessen-Martens et al. Some scientists believe that widespread biological nitrogen fixation, achieved mainly through the use of cover crops, is the only alternative to industrial nitrogen fixation in the effort to maintain or increase future food production levels (Bohlool et al.
As well as bringing nitrogen into agroecosystems through biological nitrogen fixation, cover crops known as “catch crops” are used to retain and recycle soil nitrogen already present. The catch crops take up surplus nitrogen remaining from fertilization of the previous crop, preventing it from being lost through leaching (Morgan et al. The nitrogen tied up in catch crop biomass is released back into the soil once the catch crop is incorporated as a green manure or otherwise begins to decompose (Thomsen and Christensen 1999).
Soil Quality Management
Cover crops can improve soil quality by increasing soil organic matter levels through the input of cover crop biomass over time.
Although cover crops can perform multiple functions in an agroecosystem simultaneously, they are often grown for the sole purpose of preventing soil erosion. Dense cover crop stands physically slow down the velocity of rainfall before it contacts the soil surface, preventing soil splashing and erosive surface runoff (Romkens et al. Additionally, vast cover crop root networks help anchor the soil in place and increase soil porosity, creating suitable habitat networks for soil macrofauna (Tomlin et al.
Soil quality is managed to produce optimum circumstances for crops to flourish.
Water Management
By reducing soil erosion, cover crops often also reduce both the rate and quantity of water that drains off the field, that would normally pose environmental risks to waterways and ecosystems downstream (Dabney et al. Cover crop biomass acts as a physical barrier between rainfall and the soil surface, allowing raindrops to steadily trickle down through the soil profile. Also, as stated above, cover crop root growth results in the formation of soil pores, which in addition to enhancing soil macrofauna habitat provides pathways for water to filter through the soil profile rather than draining off of the field as surface flow.
Just before cover crops are killed (by such practices including mowing, tilling, discing, rolling, herbicide application) they contain a large amount of moisture. When the cover crop is incorporated into the soil, or left on the soil surface, it often increases soil moisture. In agroecosystems where water for crop production is in short supply, cover crops can be used as a mulch to conserve water by shading and cooling the soil surface.
While cover crops can help to conserve water, in temperate regions (particularly in years with below average precipitation) they can draw down soil water supply in the spring, particularly if climatic growing conditions are good. In these cases, just before crop planting, farmers often face a tradeoff between the benefits of increased cover crop growth and the drawbacks of reduced soil moisture for cash crop production that season.
Weed Management
Thick cover crop stands often compete well with weeds during the cover crop growth period, and can prevent most germinated weed seeds from completing their life cycle and reproducing. If the cover crop is left on the soil surface rather than incorporated into the soil as a green manure after its growth is terminated, it can form a nearly impenetrable mat. This is often termed the cover crop smother effect (Kobayashi et al.
Some cover crops suppress weeds both during growth and after death (Blackshaw et al. During growth these cover crops compete vigorously with weeds for available space, light, and nutrients, and after death they smother the next flush of weeds by forming a mulch layer on the soil surface. (2001) found that when using Melilotus officinalis (yellow sweetclover) as a cover crop in an improved fallow system (where a fallow period is intentionally improved by any number of different management practices, including the planting of cover crops), weed biomass only constituted between 1-12% of total standing biomass at the end of the cover crop growing season. Furthermore, after cover crop termination, the yellow sweetclover residues suppressed weeds to levels 75-97% lower than in fallow (no yellow sweetclover) systems.
In addition to competition-based or physical weed suppression, certain cover crops are known to suppress weeds through allelopathy (Creamer et al. In one study, rye cover crop residues were found to have provided between 80-95% control of early season broadleaf weeds when used as a mulch during the production of different cash crops such as soybean, tobacco, corn, and sunflower (Nagabhushana et al.
Disease Management
In the same way that allelopathic properties of cover crops can suppress weeds, they can also break disease cycles and reduce populations of bacterial and fungal diseases (Everts 2002), and parasitic nematodes (Potter et al.
Pest Management
Some cover crops are used as so-called “trap crops”, to attract pests away from the crop of value and toward what the pest sees as a more favorable habitat (Shelton and Badenes-Perez 2006). Trap crop areas can be established within crops, within farms, or within landscapes. In many cases the trap crop is grown during the same season as the food crop being produced.
Other cover crops are used to attract natural predators of pests by providing elements of their habitat. Findings on the relationship between cover crop presence and predator / pest population dynamics have been mixed, pointing toward the need for detailed information on specific cover crop types and management practices to best complement a given integrated pest management strategy. Researchers found that the planting of several different leguminous cover crops (such as bell bean, woollypod vetch, New Zealand white clover, and Austrian winter pea) provided sufficient pollen as a feeding source to cause a seasonal increase in Congdon populations, which with good timing could potentially introduce enough predatory pressure to reduce pest populations of citrus thrips (Grafton-Cardwell et al.
Diversity and Wildlife
Although cover crops are normally used to serve one of the above discussed purposes, they often simultaneously improve farm habitat for wildlife. The use of cover crops adds at least one more dimension of plant diversity to a cash crop rotation. Since the cover crop is typically not a crop of value, its management is usually less intensive, providing a window of “soft” human influence on the farm. This relatively “hands-off” management, combined with the increased on-farm heterogeneity created by the establishment of cover crops, increases the likelihood that a more complex trophic structure will develop to support a higher level of wildlife diversity (Freemark and Kirk 2001). In one study, researchers compared arthropod and songbird species composition and field use between conventionally and cover cropped cotton fields in the Southern United States. The cover cropped cotton fields were planted to clover, which was left to grow in between cotton rows throughout the early cotton growing season (stripcover cropping). During the migration and breeding season, they found that songbird densities were 7-20 times higher in the cotton fields with integrated clover cover crop than in the conventional cotton fields. Arthropod abundance and biomass was also higher in the clover cover cropped fields throughout much of the songbird breeding season, which was attributed to an increased supply of flower nectar from the clover. The clover cover crop enhanced songbird habitat by providing cover and nesting sites, and an increased food source from higher arthropod populations (Cederbaum et al.
Conclusions
Cover crops have an unparalleled range of potential to improve the sustainability of agroecosystems.
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